1. Field of the Invention
The present invention generally relates to an undershoot eliminating circuit for eliminating an undershoot of a pulse waveform, and more particularly to an undershoot eliminating circuit which provides a high impedance when the impedance of a pulse transmitter which is inactive (transmits no signal) is measured. Furthermore, the present invention is concerned with a pulse transmitter having such an undershoot eliminating circuit.
2. Description of the Related Art
Recently, there has been a requirement to transmit data at a high bit rate. For this requirement, data is converted into a high-bit-rate digital pulse waveform and transferred between communication devices. There has also been a requirement to simplify transmission lines between communication devices. To satisfy this requirement, a bus is shared by a plurality of communication devices. In order to realize the bus sharing communications, a transmitter output pulse mask is defined by, for example, CCITT Recommendation I. 430 (see FIG. 12/I. 430), the disclosure of which is hereby incorporated by reference. The pulse waveform should be within the pulse mask. Further, in order to realize the bus sharing communications, it is required that each transmitter has a high impedance to substantially isolate each transmitter from the shared bus when each transmitter is inactive or does not transmit any signal. According to the CCITT Recommendation I. 430, when each transmitter is inactive, the output impedance thereof shall exceed the template in FIG. 10/I. 430 in the frequency range of 2 kHz to 1 MHz. That is, it is required that the transmitter of each communication device connected to the shared bus meets both, the mask requirement when it is active and the impedance requirement when it is inactive.
FIG. 1A shows a pulse transmitter that employs a transformer. A pulse transmitter 100 shown in FIG. 1A is connected across a load 2 having a load resistor RL via interface points A and B, and includes a pulse transformer driving circuit 11 and a pulse transformer 12. The pulse transformer 12 has a turns ratio N, a primary inductor (winding) LP, and a secondary inductor (winding) LS. The secondary winding LS is connected to a transmission line functioning as a shared bus.
FIG. 1B is a circuit diagram showing an equivalent circuit of the pulse transmitter shown in FIG. 1A obtained when the pulse waveform has a peak. At this time, a switch S1 of the pulse transform driving circuit 11 is ON, while a current IR flows through the load resistor RL (RL/N.sup.2), and a current LP flows through the primary winding of the pulse transformer 12. The ON state of the switch S1 indicates that the pulse transmitter 100 is transmitting a pulse. The OFF state of the switch S1 indicates that the pulse transmitter 100 is inactive or transmitting no signal.
FIG. 2A shows a waveform of the pulse developed across the load resistor RL (RL/N.sup.2) in the equivalent circuit shown in FIG. 1B) when the switch S1 is initially ON and then turned OFF at time T. The instance the switch S1 turns OFF, a reverse voltage develops across the load resistor RL. This reverse voltage causes an undershoot of the pulse waveform output by the pulse transformer 12.
The manner in which the above problem comes about is as follows. The current IL is continuously passing through the primary inductor LP of the pulse transformer 12 until a time T. The instance the switch S1 turns OFF, the current IL is forced to flow continuously because of the magnetic characteristics of the pulse transformer 12. However, the switch S1 is turned OFF at this time, and the equivalent circuit of the pulse transmitter 100 is changed to a closed circuit as shown in FIG. 2B. Hence, the current IL forced to flow in the primary inductor LP develops a reverse voltage across the load resistor RL (RL/N.sup.2). This reverse voltage corresponds to an undershoot of the pulse waveform.
In order to eliminate the above undershoot, an undershoot eliminating circuit 131 is connected in parallel to the primary winding LP of the pulse transformer 12, as shown in FIG. 3A. The undershoot eliminating circuit 131 includes a capacitor C connected in parallel to the primary winding LP. As shown in FIG. 3B, a voltage E is applied across the capacitor C until the aforementioned time T. Hence, the capacitor C is charged so that it has polarities as shown in FIG. 3B. At the time T, the switch S1 turns OFF. At this time, the pulse transmitter 12 shown in FIG. 3A has an equivalent circuit shown in FIG. 3C. The capacitor supplies the load resistor RL (RL/N.sup.2) with a backward current IC reverse to the current IL forced to pass through the load resistor RL (RL/N.sup.2) due to the function of the primary inductor LP. Hence, the current IL is canceled by the current IC and no current passes through the load resistor RL (RL/N.sup.2). In this manner, the undershoot can be eliminated from the pulse waveform.
FIG. 4A shows an equivalent circuit of the pulse transmitter shown in FIG. 3A obtained when the impedance of the inactive pulse transmitter is measured. In this measurement, an impedance meter 3 is connected in parallel to the pulse transformer 12 via the interface points A and B. The impedance meter 3 is made up of a synthesizer 31, and a level meter 32. The synthesizer 31 comprises a resistor R and an oscillator OC connected in series. The synthesizer 31 generates a sine wave having a predetermined frequency and a predetermined power level. The level meter 32 measures a voltage level developed across the load (in this case, the interface points A and B of the pulse transmitter 100). The impedance of the load (the pulse transmitter 100 in this case) is calculated from a voltage level indicated by the level meter 32. The frequency and the voltage of the sine wave generated by the synthesizer 31 can be varied, and the impedance of the load connected to the impedance meter 3 can be obtained by measuring a current passing through the impedance meter 3.
When the switch S1 is OFF, the impedance Z of the pulse transmitter 100 is written as follows: EQU Z=1/{(1/j.omega.LS)+j.omega.C.times.N.sup.2 } (1)
where .omega. is equal to 2.pi.f, f denotes the frequency of the sine wave, and LS denotes the inductance of the pulse transformer 12.
When the frequency is high, the expression (1) can be made to approximate to expression (2): EQU Z=1/j.omega.C.times.N.sup.2.
It can be seen from the expression (2) that there is a tendency such that as the capacitance of the capacitor C, provided for eliminating an undershoot of the pulse waveform, increases or as the measurement frequency increases, the impedance of the pulse transmitter 100 decreases.
FIG. 4B shows the aforementioned impedance characteristic of an NT (Network Termination unit) pulse transmitter for use in the bus connection prescribed in the CCITT Recommendation I. 430. The above impedance characteristic is indicated by symbol P. It is required that the pulse transmitter has an impedance exceeding the impedance defined by the impedance characteristic curve P in the range of 2 kHz to 1 MHz.
Recently, there has been a requirement to use a compact transformer in order to miniaturize the pulse transmitter and reduce the production cost. In general, such a compact transformer has an impedance characteristic indicated by lines (1) shown in FIg. 4B. That is, the compact transformer does not have a large impedance margin with respect to the impedance characteristic prescribed by the CCITT Recommendation I. 430. When the capacitor C for eliminating an undershoot of the pulse waveform is connected as shown in FIG. 3A, the impedance characteristic of the pulse transmitter 100 changes as shown by a line indicated by (2) in a high-frequency range (the impedance characteristic obtained in a low-frequency range is omitted for the sake of clarify). It can be seen from FIG. 4B that the impedance characteristic in the high-frequency range does not meet the impedance characteristic prescribed by the CCITT Recommendation I. 430. As the capacitance of the capacitor C for eliminating an undershoot of the pulse waveform increases in order to meet the pulse mask defined by the CCITT Recommendation I. 430, the impedance margin with respect to the prescribed impedance characteristic decreases and finally the impedance characteristic of the pulse transmitter fails to meet the prescribed impedance characteristic.
In the conventional technique, the capacitance of the capacitor is manually adjusted during the production process so that both the pulse mask requirement and the impedance characteristic requirement are satisfied. However, this needs a large number of production steps and test steps.